Mr. Monk and The Two Assistants (Mr. Monk #4) by Lee Goldberg

Fourth in the all-new series of original mysteries starring Adrian Monk, the brilliant investigator who always knows when something's out of place.

Now that her ne'er-do-well husband has been arrested for murder, Monk's former assistant, Sharona, is ready to reclaim her place in Monk's extremely well-ordered life. But his current assistant, Natalie, is not pleased with this turn of events.

While Monk tries to maintain a delicate balance between the two women, he discovers a few snags in the case against Sharona's husband. And with other murders to investigate, Monk realizes he may be up against a killer who not only understands him, but is one step ahead.